Jaro,
I recall in the thermal regime with 99.999% Li7 (IIRC not quite sure how many 9's) the production of tritium from Li6(n,t) was about equal to the production from Li7. I think
The reaction of interest is (n,t) and Li6(n,t) is about four orders of magnitude larger than Li7(n,capture).

I don't think natural Li is a candidate for fast or slow reactors unless you are trying to produce tritium.

Superposing the crossection graph on the MSFR flux profile, it becomes clear that the Li7(n,TOT) trace is mostly inelastic scattering: It leaves a big dip in the flux at the corresponding energy.
Turns out the extra tritium comes not from (n,n+T)alpha, but from (n,2n) reactions turning Li7 into Li6, which then goes on to Li(n,T)alpha.
The Li7(n,2n)Li6 reaction only starts around 8.5MeV, so not a great deal of them going on at once in the MSFR spectrum.....

Stretching out the fast part of the MSFR spectrum and superposing crossection traces for Li6, Li7, F19 and a few others, it's clear that the big dips below about 500keV are due to fluorine and lithium (which we already knew -- just a clearer picture here...)

If we were able to take out the inelastic scattering of F19 and Li7, then the MSFR spectrum might look something like this.

Not much effect above about 650keV, and huge holes below that.

Using the "clean" MSFR spectrum, an approximation could then be sketched for other salts - say, with Li7 and/or F19 replaced by things like NaF, NaCl, U/ThCl4, RbCl, etc. -- keeping in mind that the fuel itself brings lots of halide atoms to the mix, and that heavier carrier salts will tend to push the flux distribution peak slightly to the right (higher energy), as neutron moderation by elastic collisions is also reduced.....

jaro wrote:
Turns out the extra tritium comes not from (n,n+T)alpha, but from (n,2n) reactions turning Li7 into Li6, which then goes on to Li(n,T)alpha.
The Li7(n,2n)Li6 reaction only starts around 8.5MeV, so not a great deal of them going on at once in the MSFR spectrum.....

ORNL I think was targeting 99.995% 7Li. I see from my notes that the equilibrium point is 13 moles/GWe-yr tritium produced (6Li consumed) and so we must also have 13 moles/GWe-yr of 7Li(n,2n) -> 6Li. The equilibrium is a 99.9992% 7Li. I thought it was due to thermal neutrons not >8MeV neutrons. A cross section ratio for 7Li(n,2n)/6Li(n,gamma) around 8e-6 would accomplish this. I presume this analysis was done in a thermal spectrum. I'm not sure how it would change with a faster spectrum.

With Cl35/37 replacing F19 and Na23 replacing Li7, the MSFR spectrum would probably look something like the green trace below: essentially no significant dips down to about 100keV (there are many small narrow ones below ~300keV).
Even better would be carrier-free uranium chlorides.
Strangely, there doesn't seem to be any NNDC data on K39/41(n,NON).

I started this topic because my/our goal is promote an affordable/practical way to implement Weinberg's "technical fix" for Mankind's fossil fuel addiction & a decade's worth of research by many EU workers strongly suggests that a fluoride salt based "isobreeder" is a promising way to accomplish it (the key breakthrough being the fact that a particular - one of many possibilities - MSFR implementation scenario would simultaneously 1) eliminate the need for a reactor's owner/operator to implement a complex/expensive/fussy fuel salt separation scheme, 2) generate almost no TRU waste, & 3) not generate several hundred (thousand?) tonnes of really crapped up graphite waste every few years). Rehashing old arguments about the merits of chloride vs fluoride, NaF vs LiF, etc., may be lots of fun but tends to distract us from thinking about what really still needs to be worked out; i.e., the design of a "maintainable" reactor. GNEP went down in flames because the USA's NE experts & their supporters in DOE decided to keep pushing on the same old rope (LMFBR) that had repeatedly failed to convince USA decision makers that implementing sustainable nuclear power would be worth the candle. Those experts/supporters need to be reeducated & we're not going to accomplish that by losing focus.

Darryl,
Recently good progress has been made toward making a graphite free MSR viable. Certainly it is attractive to avoid the graphite waste and required replacement and concerns about a slow positive thermal coefficient of reactivity.

Removing 58Ni looks to be a promising way to address helium embrittlement of the first wall (though I think making the first wall swappable is also quite viable).

Tritium containment is a concern that needs work. Using a different salt might avoid that problem all together. However, switching to a chloride salt means losing lots of experience so I think that cure is worse than the ailment.

Even if we stay with FLiBe (I think this is the right answer) we still need to understand the tritium generation and think through how the tritium gets captured before it gets to the atmosphere. I think we have numbers on the tritium generation in a thermal spectrum. I'm not so sure we have it for a fast spectrum. Replacing the 7Li will mean higher tritium production levels unless the 6Li levels in the new salt is lower than the equilibrium 6Li levels. In thermal spectrum the equilbrium 6Li level is 6x lower than the new salt 6Li levels (at 99.995% 7Li).

Lars wrote:Darryl,
...we still need to understand the tritium generation and think through how the tritium gets captured before it gets to the atmosphere. I think we have numbers on the tritium generation in a thermal spectrum. I'm not so sure we have it for a fast spectrum. Replacing the 7Li will mean higher tritium production levels unless the 6Li levels in the new salt is lower than the equilibrium 6Li levels. In thermal spectrum the equilbrium 6Li level is 6x lower than the new salt 6Li levels (at 99.995% 7Li).

I'm not going to worry about stuff that the EU's folks don't seem to be too concerned about for good reason - just how difficult would it be separate tritium from a 600C fluoride-based molten salt?

To change the subject, here's the kind of "technical" paper that this country's NE movers & shakers are presenting at international GEN IV conferences these days.

Separating the tritium from the off-gas isn't hard. The tricky part is grabbing it before it gets through the HX's. Hot hydrogen travels through hot metal. In a thermal regime we generate 2400 Curies of Tritium per day in a new reactor (and about 1/6th of that at equilibrium). I don't know what happens in a faster reactor. The legal limit is 10 curies/day but LWR emit much much lower than this. Vermont Yankee was abandoned after a tritium leak that was pretty minor so the anti's know how to scare people with tritium. I don't expect it will be politically feasible to leak tritium at the legal limit. So we have to be effective at grabbing it before it gets into the atmosphere. The first collection point is the off-gas system. Any tritium we can get to go there we should be 100% effective at collecting since the outer seals of the offgas system presumably will be at low temperatures where the metal doesn't leak hydrogen. We might gather anywhere from 30 to 90% of the tritium this way.
Next we grab the tritium that penetrates the primary HX. This tritium is found in the secondary salt. Some secondary salts will chemically trap most of the tritium that comes from the fuel salt but the mechanism is not understood. In particular, we do not know what percentage of the tritium gets trapped (and I don't know how we remove the chemically trapped tritium from the secondary salt). We need to remove more than 99% and possible 99.9% of the tritium here. If it gets past the secondary salt and into the steam then the working assumption is that it will make its way to the atmosphere.
These are not insurmountable challenges. But if you are in a faster spectrum then the thermal neutron absorption that eliminated some fluoride salts from consideration is not so big an issue anymore. And even if we stay with FLiBe we need to understand how much tritium is being generated.

Lars wrote:Tritium containment is a concern that needs work. Using a different salt might avoid that problem all together. However, switching to a chloride salt means losing lots of experience so I think that cure is worse than the ailment.

You gotta ask yourself: What's less expensive and less of a PR hassle -- developing a Li7 production industry from scratch and dealing with all the tritium, or spending a bit on materials R&D for dealing with chloride salts ?

Incidentally, chloride salt is what's being developed for fuel reprocessing already anyway, so it's not as if it was something totally out of the blue.....

Also, the French MSFR concept specs LiF-UF4/PuF3 salt (not FLiBe), which has about the same melting point as UCl4 (UCl3/UCl4 eutectic might be a bit lower).

Lars wrote: If it gets past the secondary salt and into the steam then the working assumption is that it will make its way to the atmosphere. .

If the first HX is made of permeable stuff so it gets to a second HX that's also made of H permeable stuff, it'll end up in the helium that we're blowing through our turbines. That gas in turn should be easy to clean up by running a slip stream over copper II oxide which will convert any H(or T) in it to water vapor which could then be quantitatively adsorbed by P2O5.